The present invention relates to a method of ultrasonic pickling and a pickling apparatus for steel strips and steel sheets or nonferrous metal sheets and the like, and aims particularly at improving the acid resisting properties of ultrasonic diaphragms to be installed in an acid solution of hydrochloric acid, sulfuric acid or the like.
So far, pickling treatment by hydrochloric acid or sulfuric acid has been applied to removing oxided scale adhering to steel sheet surfaces or cleaning the surfaces in a production process of steel strips and steel sheets in the production field of iron and steel. AS for this pickling method, there are a method of removing scale by simply dipping steel sheets or steel strips in an acid solution and another method of removing scale by further installing ultrasonic diaphragms in the acid solution and taking advantage of the ultrasonic vibrations. Various technologies for installing ultrasonic diaphragms in an acid solution have been proposed. For example, in Japanese Unexamined Patent Publication No. S62-210082, disclosed is a technology aiming at improving acid resisting properties and wear resistance by making diaphragms composed of ultrasonic vibrators with a metallic material containing a specific proportion or more of Ta and/or Nb. Further, in Japanese Unexamined Patent Publication No. H4-187749, disclosed is a technology aiming at improving corrosion resistance and cavitation erosion resistance in an acid solution by adding Pd in a Tixe2x80x94Al-V system alloy having a reduced amount of Fe, hot rolling it, and then applying solution heat treatment and aging treatment to it under specific conditions. Furthermore, disclosed are: in Japanese Unexamined Utility Model Publication No. H3-103256, a two-tank type ultrasonic diaphragm unit wherein an ultrasonic diaphragm containing a vibrator itself is mounted in a sealed protective casing composed of acid-resistant resin, water supply and drainage pipes are connected to the protective casing, and water is filled therein; and, in Japanese Unexamined Utility Model Publication No. H3-11060, a pickling apparatus wherein a pickling tank consists of a dual structure having an outer tank and an inner tank, an acid solution is filled in the inner tank and a liquid other than the acid solution is filled between the inner tank and the outer tank, and a plurality of ultrasonic vibrators are attached to the side walls of the outer tank. Further yet, in Japanese Unexamined Patent Publication No. H10-1800, proposed is a scale removing method and an apparatus combining a known electrolytic pickling method with an ultrasonic cleaning method for steel strip surfaces in one compound scale removing operation to remove scale on the surfaces of hot-rolled or cold-rolled steel strips.
As shown in FIG. 4, in a conventional pickling apparatus for a hot-rolled steel strip, scale dissolving treatment has been applied to the surfaces of a hot-rolled steel strip 3 by passing the hot-rolled steel strip through a pickling tank 1 filled with an acid solution 2 and continuously dipping the strip therein. In recent years, being developed are technologies of applying ultrasonic waves to the surfaces of a hot-rolled steel strip 3 via an acid solution 2 by installing ultrasonic diaphragms 4 in a pickling tank 1 to improve scale removing efficiency for the purpose of enhancing productivity. However, presently none of such technologies has been commercially realized, since the diaphragms 4 deteriorate due to the acid and their life is shortened. To overcome this problem, being extensively developed are technologies for prolonging the life of diaphragms 4 by giving acid resisting properties thereto. For example, proposed are: a method of making diaphragms 4 using an acid resistant metal in the pickling apparatus shown in FIG. 4; a method of installing an ultrasonic diaphragm 4 in a sealed protective casing 12 composed of acid-resistant resin and filling the protective casing 12 with water 13 in the pickling apparatus shown in FIG. 5; and a method of passing a hot-rolled steel strip 3 through the interior of a dual casing 12 filled with an acid solution, filling the exterior of the protective casing 12 with water 13, and then installing ultrasonic diaphragms 4 outside a pickling tank 1 in a pickling apparatus shown in FIG. 6.
However, there are following problems in these methods: in the case of Japanese Unexamined Patent Publication No. S62-210082, the equipment cost becomes very high and, moreover, the effect of accelerating pickling can not be obtained; in the case of Japanese Unexamined Patent Publication No. H4-187749, the effect of accelerating pickling can not be obtained either, the cost of the ultrasonic diaphragm becomes high, since a metal having corrosion resistance to high temperature acid is used, and the ultrasonic waves are attenuated, since a sealed casing is used; and in case of the scale removing apparatus disclosed in Japanese Unexamined Patent Publication No. H10-1800, the life of the ultrasonic diaphragm is short and the cost becomes high.
The object of the present invention is, in view of the aforementioned problems, to provide a low cost ultrasonic diaphragm having acid resistance and capable of securing the intensity of ultrasonic waves in ultrasonic pickling used for removing scale on hot-rolled steel sheets. As a result of an extensive study to achieve the object, the present inventors found that it was possible to provide a low cost ultrasonic diaphragm having acid resistance and capable of securing the intensity of ultrasonic waves by applying electric anticorrosion treatment to the surfaces of an ultrasonic diaphragm. The present invention is completed based on this finding and the gist of the present invention is as follows:
(1) a method of ultrasonic pickling, characterized by impressing a protective potential of baser than the corrosion potential of an ultrasonic diaphragm installed in a pickling solution on the ultrasonic wave imposing means and carrying out pickling while supplying corrosion preventive current to a corrosion preventive current imposing means whose negative electric terminal is connected to the ultrasonic wave imposing means and whose positive electric terminal is connected to said pickling solution or a travelling strip material;.
(2) a method of ultrasonic pickling according to the item (1), characterized by impressing a protective potential of not less than xe2x88x92300 mV baser than the corrosion potential of an ultrasonic diaphragm on said ultrasonic wave imposing means and supplying corrosion preventive current of not less than 100 mA/m2 to said ultrasonic diaphragm;
(3) an apparatus for ultrasonic pickling, characterized by having an ultrasonic wave imposing means installed in a pickling solution and a corrosion preventive current imposing means whose negative electric terminal is connected to said ultrasonic wave imposing means and whose positive electric terminal is connected to said pickling solution or a travelling strip material;
(4) an apparatus for ultrasonic pickling according to the item (3), characterized by hermetically sealing the circumference of a space formed by ultrasonic vibrators, the wiring for electric anticorrosion and an ultrasonic diaphragm with a box made of resin or metal.